Hydrocarbon fuels that must be pressurized when at atmospheric temperature in order to retain them in the liquid state are referred to as "liquefied petroleum gases". (LPG. Two well known examples of such fuels are Butane and Propane. Propane has been used for many years in internal combustion engines and was particularly popular during the late 1960's and early 1970's when the increasing price of gasoline and more stringent requirements on control of fuel emissions increased interest in alternatives to gasoline. LPG fuels continue to be less expensive and are cleaner burning fuels than gasoline. Propane has been the fuel of choice over butane for reasons that will follow.
The standard engine using gasoline simply utilizes a pump to transfer liquid gasoline from a nonpressurized storage tank to a carburetion system.
The state-of-the-art LPG powered engine utilizes a fuel which is stored as a liquid and converted to a vapor before its induction into the engine. The stored fuel is maintained as a liquid in the storage area by virtue of a sufficiently high vapor pressure in the closed storage area which is a function of the ambient temperature. It is this pressurization of the storage tank which provides for fuel delivery to a vaporizer unit which consists of a pressure regulator and a heater. The vaporizer releases the fuel as a vapor which is then drawn by engine vacuum into a device for mixing vaporized fuel and air similar to a standard gasoline carburetor.
Because the fuel enters the engine as heated vapor, a certain amount of air is displaced which lowers density of the intake air/fuel mixture and results in a loss of power. In addition, both LPG hydrocarbons contain less heat energy (BTU/gal) than gasoline. Propane has a much lower boiling point (-40.degree. F.) and much higher vapor pressure than butane whose boiling point is 31.degree. F. In low temperature environments, pure Butane does not provide sufficient vapor pressure to deliver fuel to the engine using the pressurized fuel delivery system of the prior art.